The present invention relates to time base sweep circuitry and particularly to such circuitry which is adapted to provide a linear sweep with fast recovery.
In a cathode ray oscilloscope or similar device, a varying signal can be displayed as a function of time by applying the signal to vertical deflection means of a cathode ray tube, while a linear sweep signal or time base signal is applied to the horizontal deflection means. Between "sweeps", the horizontal deflection means provides retrace back to the starting point at the left hand side of the cathode ray tube screen, the electron beam generally being blanked out during such retrace period. The horizontal deflection is then held at a stable starting position awaiting the occurrence of the next repetition of the input signal that is to be displayed.
A suitable form of time base circuitry for operating the horizontal deflection means of the cathode ray tube involves a timing capacitor and means for supplying a substantially constant current to the capacitor whereby a linear ramp voltage is provided across the capacitor for application to the aforementioned horizontal deflection means. In a typical circuit, a current switch alternately connects a secondary current path to the timing capacitor for discharging the timing capacitor at the end of each sweep (to provide retrace) and further for establishing bias during baseline stabilization awaiting the beginning of the next sweep. At the beginning of the next sweep, the additional current path is interrupted whereby the timing capacitor is allowed to charge up once again in a linear manner. Large current requirements were imposed upon this secondary current path in order to bring about rapid retrace. Thus, the current from the capacitor during the retrace period may well be five to ten times as much as the current provided thereto during the linear sweep. Unfortunately, at the beginning of a following sweep after retrace, this fairly large current had to be interrupted, i.e. the switch had to be turned off, whereby undesirable transients appeared in the circuit as a consequence of the current interruption and the presence of parasitic inductances and capacitances in the circuit. The result tended to be a non-linear sweep signal especially for very high sweep speeds, as well as interference with the vertical channel signals of the oscilloscope.
The above problem is aggravated when many selectable sweep speeds are desired, while using a comparatively small number of switchable timing capacitors. To obtain a wide range of sweep speeds with few capacitors, it is necessary to apply a large range of selectable currents to the timing capacitors. This inevitably means extremely high current during some retrace periods since fast recovery is required after a sweep is finished. The interruption of these high currents aggravated the problem of undesired transients.
Furthermore, undesirable transients can also occur when the aforementioned current switch turns "on" for discharging the timing capacitor at the beginning of retrace. It would be desirable to eliminate the problems occurring both at the beginning and at the end of the time base sweep.
A further problem with this kind of timebase generator is associated with the stability of circuitry for maintaining an accurate baseline value, or initial starting point, for the time base sweep signal. It would be desirable to establish a very stable initial baseline value.
An additional problem present with the aforementioned type of timebase generator is associated with the current source utilized in charging the timing capacitor during the time base sweep. The current source generally involves a semiconductor device which itself exhibits an inherent capacitance, and the charging current for the latter capacitance subtracts from the current as would otherwise be supplied to the timing capacitor. It would be of advantage to remove this source of inaccuracy.